Power plant installations using coal as fuel and consisting of combinations of gas turbines, steam turbines and electric generators are known. It is also known that the heat output from a gas turbine can be utilized in a steam turbine circuit which also can produce part of its steam by utilizing the sensible heat of the fuel gases produced by and in the coal gasifier. Combination power plants of the basic structure described are known in various embodiments (see, for instance, "Energiewirtschaftliche Tagesfrage,"1984, pp. 802-808). By virtue of having the main power plant and the coal gasifier located at one place, there are substantially advantages: on the one hand the sensible heat of the produced fuel gases and the gasification operation can be used in the form of high pressure steam in the steam turbine, and on the other hand the low-pressure steam and the medium-pressure steam from the steam turbine can be used for the drying and the gasification of the coal.
In the following disclosure, the term "main plant" means generally a power plant with an output of at least 300 MW, i.e. a central power plant serving a large area, although the concept applies most broadly to any large power plant with an output greater than its satellite plant or plants.
However, the overall efficiency of such power plants as well as their reliability require improvement. It is necessary when operating them to set fuel consumption according to the amount of electricity needed, so that the plant must be dimensioned to handle large peak loads at which it is highly efficient, but must normally run at a much less efficient rate.
The standard procedure for overcoming this problem as discussed in U.S. Pat. Nos. 3,873,845 and 4,085,578 of Osthaus and Kydd, respectively, is to stockpile the excess gas produced during periods of low electricity needs, and burn it when demand is higher. Such storage is a problem and still often leads to running the gasifier at a relatively low rate when its efficiency would be boosted considerably if set to burn more fuel.